Of the aromatic C8 isomers, including the three xylene isomers and ethylbenzene, paraxylene is of particularly high value since paraxylene is useful in the manufacture of synthetic fibers and resins. Refinery and chemical plant streams containing the aromatic C8 isomers typically contain, at thermodynamic equilibrium, only about 22-24 wt % paraxylene, based on the weight of the xylene isomers in the stream. Separation of paraxylene from the other C8 isomers requires superfractionation and/or multistage refrigeration steps and/or adsorptive separation, all of which are energy intensive. There is a need to provide processes for producing paraxylene in more efficient ways, such as in higher selectivity than can be obtained from refinery and chemical plant streams.
One known method for producing paraxylene selectively involves the alkylation of toluene and/or benzene with methanol and/or DME (dimethylether) over a solid acid catalyst. Selectivities to paraxylene in excess of 90 wt % (based on total C8 aromatic product) have been reported by reacting toluene with methanol in the presence of a catalyst comprising a porous crystalline material, preferably a medium-pore zeolite and particularly ZSM-5, having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa). See U.S. Pat. Nos. 6,423,879 and 6,504,072.
WO 99/38823 reported a reactive distillation process comprising the contact of toluene with a methylating agent in the presence of a zeolite in a reaction/distillation column produces, as a side product, DME, which can be recycled (with unreacted methanol) to extinction in the process. The process operates at no greater than 320° C.
It has recently been discovered that the alkylation of benzene and/or toluene with methanol can also result in the production of a variety of oxygenates, in addition to DME, but also other oxygenate by-products. See for instance U.S. patent application Ser. No. 13/487,651. According to the invention described in Ser. No. 13/487,651, the concentration of phenolic impurities in a xylene stream produced by alkylation of benzene and/or toluene with methanol can be reduced to trace levels, e.g., below 0.1 ppmw, by one or more washing treatments with an aqueous solution of a base. The resultant treated xylene stream, if necessary after water washing to remove any phenate-containing solution, can then be recycled to the xylene splitter to generate additional para-xylene or can be used as a solvent.
Recently, a process for the production of paraxylene selectively by: (i) reacting of toluene and/or benzene with methanol in the presence of a suitable catalyst under appropriate conditions to process stream comprising paraxylene in higher than equilibrium amounts; (ii) contact of said process stream comprising paraxylene in higher than equilibrium amounts with a suitable adsorbent to remove phenol, said phenol having been produced in (i), or is present in the feedstream of toluene and/or benzene and/or alkylating agent (methanol and/or DME), or any combination thereof, to provide a product stream having a lower concentration of phenol than said process stream, has been described in U.S. Provisional Patent Application No. 61/653,698.
It has also recently been discovered that xylenes produced by alkylating toluene and/or benzene with an alkylating agent comprising methanol and/or DME over a solid acid catalyst contain small quantities of styrene, which, if not removed, could cause operability problems for downstream paraxylene recovery processes, or even further, in processes using paraxylene, such as the production of terephthalic acid, and derivatives thereof, including polyester fibers, films, and the like.
Several characteristics of the xylene produced in this manner make styrene removal challenging. The desired product, paraxylene, is present at higher-than-equilibrium concentration. The catalyst used to remove styrene must therefore show minimal xylenes isomerization activity. The catalyst must also minimize formation of benzene, which also can have detrimental effects on downstream processing. Furthermore, as already mentioned, the product contains a variety of oxygenates, such as phenol. Moreover, olefinic compounds may enter the alkylation reaction system via the feedstream of toluene such as catalytic reforming units, which are a source of toluene for the aforementioned alkylation reaction. These and other problems make the treatment of the product stream from the alkylation of benzene and/or toluene in the presence of an acid catalyst difficult.
U.S. Pat. No. 4,795,550 teaches converting trace quantities of olefinic impurities to nonolefinic hydrocarbons by contacting an aromatic process stream from an alkylation reaction with a solid catalyst composite comprising a crystalline aluminosilicate zeolite and a refractory inorganic oxide. Faujasite is mentioned as a preferred aluminosilicate zeolite and the refractory inorganic oxide can be alumina, silica-alumina, or a mixture of both.
U.S. Pat. No. 6,313,362 teaches that in an alkylation/transalkylation process, an aromatic alkylation process stream comprising polyalkylated aromatic compounds is contacted with a purification medium in a liquid phase pre-reaction step, prior to transalkylation, to remove impurities, including styrene. A large pore molecular sieve catalyst such as MCM-22 may be used as the purification medium in the pre-reaction step because of its high reactivity for alkylation, strong retention of catalyst poison and low reactivity for oligomerization under the pre-reactor conditions. The alkylation processes envisioned include the production of ethylbenzene, cumene, ethyltoluene, and cymenes.
U.S. Pat. No. 7,731,839 teaches treating an aromatic hydrocarbon feedstock having undesirable olefins including styrene with a catalyst such as MCM-22 to reduce the amount of undesirable olefins. Likewise, U.S. Pat. No. 6,005,156 teaches a process for the reduction of olefins and diolefins from mixtures of aromatic hydrocarbon-rich cuts by treatment in a hydrogenation zone and then treatment with clay. Although many sources of aromatic-rich hydrocarbon cuts are mentioned, a process for alkylation of benzene and/or toluene with methanol is not recognized. Similarly, U.S. Pat. No. 7,199,275 teaches treatment of a partially dehydrated aromatic feedstock containing styrene as an impurity by contacting with a first molecular sieve having a Si/Al molar ratio less than about 5 and a second molecular sieve having a Si/Al molar ratio of greater than about 5. The thus-treated feedstock is then used for an alkylation reaction of benzene with ethylene and/or propylene or translkylation reactions in the liquid phase.
Other references include FR 2295935, teaching the reduction of olefin and diene content of an aromatics-rich fraction by subjecting the fraction to an acid-catalyzed vapor or liquid-phase alkylation reaction; JP 2138137A, teaching separation of styrene from a C8 aromatics stream by selective adsorption of styrene with a modified faujasite zeolite; and JP 76026421B, teaching isolating styrene from a hydrocarbon fraction comprising styrene and ethylbenzene and/or xylene isomers by adsorption of styrene with a zeolite comprising an alkali or alkaline earth metal.
As far as the present inventors are aware, the prior art has not addressed the problem of styrene impurities in a system for alkylation of benzene and/or toluene with an alkylating agent selected from methanol, DME, and mixtures thereof, in the presence of a catalyst to produce a process stream comprising higher than equilibrium amounts of paraxylene and with the co-production of styrene.
The present inventors have surprisingly discovered a method of purifying said process stream of styrene impurities without significant loss of the desired paraxylene product or co-production of additional impurities.